This invention relates to a marine propulsion unit and more particularly to a manifold and water trap and output shaft arrangement for such an engine.
There is a very popular and rapidly growing segment of the watercraft market for a type of water vehicle called a "personal watercraft." This type of watercraft is designed primarily to be operated by a rider and accommodate a relatively small number of passengers. Frequently, the rider's area is generally open and the watercraft and rider's area is designed so as to be boarded from the body of water in which the watercraft is operating. Obviously, these vehicles are quite compact in nature.
The propulsion systems for these personal watercraft normally include an inboard mounted engine and frequently a jet propulsion unit in the form of a jet pump that is positioned on the underside at the rear of the hull. Oftentimes the engine is positioned at least in part beneath the rider's area and frequently beneath a raised straddle-type seat that runs longitudinally through the center of the rider's area.
This compact construction for the watercraft presents a number of unique design problems. Not the least of these is the layout of the exhaust system for the engine. The problem is rendered particularly acute since, as is typical with marine propulsion systems, the engine exhaust gases are discharged to the atmosphere either at, below or close to the water level at which the watercraft is operating. Thus, care must be taken to ensure that water cannot enter the engine through the exhaust system. This problem is particularly acute since this type of watercraft is very sporting in nature, and the rider and occupants expect the watercraft to capsize at times. When capsized and subsequently righted, there is a problem that water may flow through the exhaust system into the engine with obviously undesirable results.
Although these types of watercraft are often powered by two-cycle crankcase compression internal combustion engines, for a wide variety of reasons, there may be advantages in employing four-cycle engines. However, when four-cycle engines are employed, the engine configuration is generally more bulky than that of a two-cycle engine for obvious reasons. A large percentage of this bulk is created by the fact that unlike two-cycle engines, the induction system and the exhaust system for the engine is normally mounted on the cylinder head. This gives rise to an increased bulk at the top of the engine and makes engine placement, particularly under a rider's seat, difficult, if not impossible.
It is, therefore, a principal object of this invention to provide an improved propulsion unit and layout for a small personal-type watercraft.
It is a further object of this invention to provide an improved engine arrangement for such a watercraft wherein the engine's center of gravity may be kept low and the bulk of the engine reduced.
One way in maintaining a low center of gravity and moving the large bulk of the engine to a lower level where it will permit positioning of a rider's area over it is to cant the engine to one side. If this type of canting arrangement is employed, there are some certain advantages in maintaining the induction system on the high side of the engine. Not the least reason for this is the fact that the induction system generally requires servicing more frequently than the exhaust system. However, if the exhaust system is placed on the low side of the engine, then the water entry problems aforenoted may be increased.
It is, therefore, a still further object of this invention to provide an improved engine arrangement for a marine watercraft wherein the engine may be canted so that the exhaust side is low but wherein protection against water intrusion into the engine through the exhaust system is optimized.
It is a further object of this invention to provide an improved engine and exhaust system arrangement for a watercraft.
As has been previously noted, the typical propulsion system for this type of watercraft employs a jet pump that is disposed on the underside of the hull and toward the rear of the hull. Generally, it is the practice to position the engine so that its output shaft is generally aligned with the impeller shaft of the jet pump so as to facilitate coupling therebetween. However, this has several disadvantages.
First, if it is desired to cant the engine over to one side or the other from the vertical, then it is difficult to maintain the in-line relationship between the engine output shaft and the watercraft drive shaft unless the entire engine is disposed substantially on one side of the longitudinal plane. This one-side positioning of the engine is not desirable because it can adversely affect the balance of the watercraft. If, however, the watercraft engine is canted and its output shaft axis is offset from the drive shaft axis, then some form of transmission must be employed for transferring the drive. In some instances this is not a particular disadvantage because it may be desirable to provide a step-down transmission between the engine output shaft and the impeller shaft.
One reason for this is that high-performance engines normally run at a high engine speed so as to achieve a high output for a given displacement. However, marine propulsion systems such as propellers or impellers for jet pumps can be subject to cavitation if driven at high rates of speed. Therefore, there are advantages in providing a step-down transmission between the engine output shaft and the drive shaft of the watercraft.
However, to provide large speed reductions requires a relatively large transmission, since the ratio between the driving element and the driven element should be relatively large. Although offsetting of the engine output shaft relative to the drive shaft can facilitate this, the normal degree of offsetting does not provide adequate space for a suitable step-down transmission.
It is, therefore, a still further object of this invention to provide an improved engine propulsion unit layout for a watercraft and a step-down transmission therebetween.